Ion-electron image converter for use with ion microanalyzers



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[54] ION-EL C IMAGE CONVERTER FOR USE ABSTRACT: In an ion-electron converter, wherein an ion WITH ION MICROANALYZERS 5 Claims, 6 Drawing Figs.

beam is directed normally to the electron emitting target, a magnetic deflector inserted in this path, is adjusted to deflect the secondary electron beam therefrom without noticeably affecting the trajectory of the incident ion beam.

PATENTEU 05029-1970 3.551.671

' "samlurs 393 PRIOR ART Y sum 2 [1F 5 5&

ION-ELECTRON IMAGE CONVERTER FOR USE WITH ION MICROANALYZERS The present invention relates to devices wherein an incident ion beam is converted into an electron beam, the latter being received by a luminescent screen or other component, all those devices being referred to hereinafter as ion-electron converters. The term of ion-electron image converter will be restricted to the particular case where an electron image is obtained, which is the image of a physical object, and not simply a section of the electron beam corresponding to a section of the ion beam.

According to the invention there is provided an ion-electron converter comprising an envelope having a face with an aperture forming an entrance diaphragm for an incident ion beam and, in said envelope: a target, emitting electrons in response to the impact of ions, said target being located opposite said entrance diaphragm, the straight line normal to said target and passing through the center of said aperture being referred to as the axis of the converter; means, in the vicinity of said target, for concentrating said electrons into a secondary beam whose axis substantially coincides with said converter axis; and magnetic means located between said concentrating means and said entrance diaphragm for deflecting said secondary beam in a direction at an angle with said converter axis.

The converter according to the invention solves in a satisfactory manner, both where positive or negative ions are used, the problem of the location of the component designed to receive the electron beam, in particular that of the photographic recorder of an image converter. The invention will be better understood, and other of its features will become apparent, from a consideration of the ensuing description and the appended drawings in which:

FIG. 1 illustrates the principle upon which a conventional ion-electron image converter is based;

FIG. 2 is a slightly more detailed diagram, showing the difficulties encountered in recording the electronic image in a conventional image converter;

FIG. 3 illustrates, in vertical section, an ion-electron image converter in accordance with the invention;

FIG. 4 is a diagram illustrating the action of the'magnetic prism in the converter of FIG.3;

FIG. 5 illustrates a preferred embodiment of the magnetic prism of FIG. 3; and

FIG. 6 illustrates a simple and highly adequate embodiment of the invention as applied to a converter of a mass spectrometer.

In the various FIGS. corresponding elements have been given the same reference numbers.

The ion-electron image converter is currently employed in apparatuses such as the ion microscope or the Castaing and Slodzian ionic microanalyser, which apparatus operates as a selective ion microscope, in that it supplies ionic images, which are selective with respect to the nature of the atoms building up the surface of the specimen to be analyzed.

It will be remembered that in these apparatuses, the purpose of the image converter is to convert the final ionic image into an electronic image for observation or recording purposes. It is not easy to perceive ionic images on fluorescent screens, since screens of this kind have an extremely low sensitivity to ions; on the other hand, the fluorescent screen would rapidly become contaminated by the ions it receives and, where positive ions are used, the electric charge of the screen would result in an instability of the image. For this reason, a converter for converting the ionic image to an electronic image, is used, the principle of which is illustrated in FIG. 1.

The ionic image AB to .be displayed, which will be assumed to be constituted in this case by positive ions, is projected by means of an electronic lens L onto A"B" on the cathode 1 of an electrostatic emission lens. The cathode I of this lens, which is biased negatively with respect to the anode 3, repels the secondary electrons emitted under the effect of bombardment by the ion beam.

The curvature of the equipotential lines, obtained by means of the control electrode 2, which is at a slightly higher potential than the cathode, causes the electron beam to be focused so that an image ab of A"B" is formed on the fluorescent screen 4, located at the crossover of the lens L.

However, the focusing action of the emission lens also affects the incident ion beam, so that the ionic image A"B" on the cathode, is contracted relatively to the image A'B' which would be produced by the lens L on its own, and which constitutes a virtual object for the emission lens.

In order to reduce this contraction effect, it is desirable to employ a high-energy incident ion beam.

When the image to be converted is initially carried by a beam of comparatively low energy, in the order of 5 keV, for example, in an ion microanalyzer the beam therefore has to be post-accelerated (to bring its energy to 15 keV, for example, in the ionic microanalyzer).

In the ionic microanalyzer where, for reasons of convenience, the beam, before entering the converter, is propagated through a region which is at ground potential, the inner envelope of the image converter is therefore at a high potential, namely the post-acceleration potential, and this creates certain problems as concerns the photographic recording of the final electronic image.

FIG. 2 is a diagram of a device of the kind shown in FIG. 1, with the addition of a post-acceleration lens L comprising two diaphragms 7 and 6.

The potentials of the different electrodes are quoted in relation to ground potential. The values indicated correspond to a system operating with positive ions, it being assumed that the ion beam carrying the ionic image to be converted, emanates from a region at ground potential, and that its energy is only about 5 keV for example.

In those circumstances, the diaphragm 7 first traversed by the ion beam, is at ground potential, whilst the diagram 6 is at the post-acceleration potential, say I 5 kV.

At the exit from the post-acceleration lens, formed by electrodes 7 and 6, there are located the elements of the converter shown in FIG. 1, except that the electrodes 1 and 2 of the emission lens are here electrically connected to form a single component 102, the adjustment of the lens being effected here by adjustment of the distance between the cathode and the control electrode.

The projection lens L, symbolically illustrated in the diagram of FIG. 1, is a conventional three-diaphragm lens, including, in the direction of propagation of the the ion beam, the diaphragms 53, 52 and 51. The electrodes 53 and 51, are at the post-acceleration potential, as are the anode 3 of the emission lens and the fluorescent screen 4.

The center electrode 52 of the projection lens L is brought to an adjustable potential u, by means of which the ionic image can be focused on the cathode of the component 102, the latter being at the potential of -45 kV.

The electronic image which appears on the fluorescent screen 4 is observed or photographed through the medium of an optical system comprising a mirror 8 and a side port not shown in the FIG. The mirror 8, arranged between the anode 3 of the emission lens and the fluorescent screen 4, is inclined at 45 to the axis of the converter; an aperture, through which the ion and electron beams can pass, is provided therein.

A converter of this kind can equally well operate using negative ions, with suitable potentials.

In order to record the electronic image, hitherto, a conventional camera has been mounted outside of the apparatus, behind the side port, since otherwise, with a conventional converter, the following difficulties were encountered:

An aperture must be provided in the photographic film for the passage of the ion beam, and this of course impairs the image.

The luminescent screen for direct observation of the image has to be retractable.

Finally, the mechanical design of the camera, which has to be at the post-acceleration potential, is rendered extremely difficult because of the risk of glow discharges exposing the film.

FIG. 3 illustrates a vertical section of the end portion of an ion-electron converter, which embodies the improvements proposed in accordance with the present invention.

The converter assembly is contained in a vacuum-tight conductive envelope 31, which is at ground potential, and a face of which (not shown in the FIG.) builds up the entrance diaphragm (7 in FIG. 2).

The external envelope 31 includes a conductive envelope 32 sustained by an insulating member 33 and brought to the post-acceleration potential by means of a sealed lead-through 34.

The converter further comprises, following the elements (not shown in FIG. 3) corresponding to lenses I. and L of FIG 2:

a tubular screen 35 through which the incident ion beam passes;

an emissive lens built up by an anode 3 at the post-acceleration potential, and by a control electrode and cathode assembly 102, mounted on an insulating member 38 and connected to an external voltage source through the sealed lead-through 39;

a movable prismatic magnet 10 at the post-acceleration potential, which prism creates a magnetic field perpendicular to the converter axis and movable azimuthally about said axis;

a luminescent screen 11 offset in relation to the axis of the converter so as not to interfere with the ion beam.

The magnetic prism is virtually without influence upon the ion beam entering the emission lens, but it deflects the electron beam leaving the lens so that the electronic image is formed away from the converter axis, for example on the fluorescent screen 11.

This is easy to achieve, taking into account the relative energies of the ions and electrons involved, since the chargeto-mass ratio of ions is a very small fraction of the charge-tomass ratio of electrons (less than III 800 for the lightest singlecharge ions).

A mechanical control system enables the magnetic prism to be rotated about the converter axis, so that, during operation, the electronic image may be rotated about this axis in a direction which depends upon the direction of the magnetic field.

The control mechanism comprises an external control knob 12 which, through a vacuum-tight gland 13 and an insulator 14, drives the bevel gear 15 which meshes with the mating toothing 16 to which the magnetic prism 10 is rigidly locked.

This facility for transferring the electronic image successively to different points of the converter, without interrupting the operation of the system, can be employed in various combinations.

For example, it is possible to use two image positions which are 180 away from one another.

The electron image is first formed directly upon the active layer of a luminescent screen 11, the support of which is transparent. The offsetting of this screen from the aforesaid axis, makes it possible to arrange behind it and in its immediate neighborhood, a mirror 17 and a large aperture objective 19, by means of which a large part of the light which the screen emits is captured. The lens 19 is fixed to the external wall of the enclosure 32 and transmits a light beam outside the apparatus across the observation port 18 formed in the system casing 31. The light beam can be picked up by a visual observation system or by a recording system (television for example).

With the magnetic prism 10 in this position, it is possible, by observation of the electronic image, to bring into focus the ionic image.

The image is then transferred, by rotating the knob 12, into a plane symmetrical to the plane of the luminescent screen with respect to the axis of the converter. In this plane, the

camera film is located.

The camera, which is at the post-acceleration potential, is fixed through an insulator 21 to a detachable mounting 20 which, when the converter is in operation, is fitted in sealed fashion in the wall of the casing 31. The camera is built up, in conventional manner, by two spools, a supply spool and a takeup spool. The takeup spool 22 is controlled by the external knob 23 through the gland 24 and the insulator 25. The supply spool is masked, in the FIG., by the spool 22.

The photographic shutter effect will preferably be achieved by extinguishing the ion beam by means of any conventional device, for example a system of deflecting plates which can easily be housed upstream of the tubular member 35.

Thus, the magnetic prism enables the first two above mentioned difficulties to be overcome, Lee. the difficulties which were concerned with the obtention of photographic recordings inside the envelope 32.

The third difficulty is solved by the design of the converter in accordance with the invention, the design being such as to enable the use of a very simple camera of small size, all the film in which is located in a substantially equipotential region, with the consequence that the risk of glow discharges is eliminated. The insulating members by which the camera is attached to the control knob 23, are located inside the insulating casing 31, the control knob 23 being the only part disposed externally of the assembly.

In addition, an important advantage of this structure is that, compared with a conventional device, the exposure time is considerably reduced (in practice exposure times are several tens of times shorter), and this is highly pertinent in situations where the ionic image develops rapidly.

A device may be associated to the luminescent screen for carrying out quantitative microanalysis on spot regions, by means of which the ion current corresponding to a very small fraction of the surface of the object ofwhich the ionic image is being formed, can be measured and this of course is a factor of considerable interest in situations where the converter is associated with an ionic microanalyzer.

This device is schematically illustrated in FIG. 3.

The luminescent screen is provided with a small aperture 26.

Through this aperture, a thin electron beam, the intensity of which is proportional to that of the ion beam (filtered, in an ionic microanalyzer, as the charge-to-mass ratio of the ions) issuing from that small specimen area, the image of which coincides with the screen aperture.

The diameter of the aperture will for example be 0.5 mm., this corresponding, if the magnification of the converter is assumed to be 10 and the total magnification of the analyzer to be 100, to a 0.05 mm. diameter circle on the cathode of the converter and to a 5 1. diameter circle on the specimen.

The intensity of the beam is measured by means of a luminescent substance 27 coupled, in the conventional manner, to a photomultiplier 28 through an insulating light guide 29. The latter may be substituted by an optical lens system.

Another combination may for example comprise a third position for the electronic image, this third position being located at to the ones aforementioned. In a section of the electron beam corresponding to this image, a luminescent element is provided, the area of which is larger than the beam cross-sectional area, and which is likewise coupled by optical means to a photomultiplier.

In this way, it is possible to measure the ion beam intensity for any part of the surface of the specimen being analyzed and to obtain the overall mass spectrum corresponding to the full area.

FIGQ4 illustrates the operation of the magnetic prism 10 of FIG. 3, which in the case of an image converter has to be carefully designed to avoid a distortion of the electron image.

In FIG. 4, the magnetic prism is simply represented by an external contour E, F, G, H, in the plane of of the vertical section corresponding to FIG. 3, the prism being assumed to be in the position which it has in FIG. 3.

This contour delimits, perpendicularly, to the plane of the FIG, the volume within which it is necessary to create a uniform magnetic field perpendicular to the plane of the FIG. and directed, in this case, forwards. This contour is trapezoidal for reasons which will be dealt with hereinafter.

It will be seen that, in the absence of the magnetic prism, the electronic image of the ionic AB produced by the converter, would be formed at ab.

This image ab plays the part of a virtual object for the prism, the prism having the task of producing a real image of ab at the location a'b whichjs offset from the axis.

The magnetic prism is designed to reduce to a negligible value any chromatic aberration and astigmatism.

FIG. 5 illustrates, in perspective, a practical embodiment of the prism.

Its volume is delimited by the right prism having the base surfaces C, C,, 0,, D and C, C,,, D,,, D,, the point C, being masked by the bulk of the FIG. The two bases of the prism are two identical isosceles trapeziums, the shorter base lines of which are C,, D and C,,D respectively. The extensions of the nonparallel sides C, C, and D, D of the trapezium C, C D, D, meet at a point S. The angle CSD is given the value 7 defined hereinafter. I

Physically, the prism is built up by two ferromagnetic components delimited externally by the isosceles trapeziums C, D, C2, D and C,, D,, C,, D;,, these two components being connectcd with one another through two parallel-disposed rectangular permanent magnets, the parallelism being both mechanical and magnetic.

The two ferromagnetic components externally delimited by the trapeziums C, D, C D and C,, D,, C D are provided, on those faces mutually opposite one another, with two projecting portions delimiting between them a narrow space designed to give passage to the beams and corresponding substantially to the volume illustrated in section FIG. 4.

This prism is located in the apparatus in such a way that:

l. The system of axes built up by;

the intersection of the entrance face of the prism and the median plane of the aforesaid gap, saidintersection being directed towards the edge of the dihedral angle between the entrance and exit faces of the prism,

the magnetic field, and

that axis of the converter directed in the same direction in which the electrons flow,

forms a direct trihedral angle;

2. the entrance face of the prism is inclined in relation to a plane corresponding to a right section through the converter, at an angle which will be explained. in greater detail hereinafter.

The electrons issuing from the emission lens have a certain energy dispersion and this means that after deflection of the beam by the magnetic prism, chromatic" aberration occurs in the image.

This aberration will be the less marked the smaller the angle of deflection a of the center ray of the electron beam.

In order that the image a'b shall be stigmatic, the radial and transverse focal lines of the prism must coincide. For this, the inclination of the entrance and exit faces of the prism can be adjusted. It can be shown theoretically (using formulas based upon Cotte's theory for example) that in order to achieve stigmatism in the case where a is small, the angle made between the entrance face of the prism and a plane perpendicular to the center ray of the input beam, and the angle made between the exit face of the prism and a plane perpendicular to the central ray of the exit beam, should respectively be equal to +a/4 and to -a/4, hence the trapezoidal contour of the magnetic prism.

In practice, these two angles are given the same absolute value, B, which is near a/4, and is adjusted in order to obtain the best result from the point of view of both stigmatism and absence of distortion.

Once the angle )9 is determined, it is not difficult to see that the angle 7 (FIG. 5) should be equal to 2B.

It is worthy of reminder here that the term magnetic prism is meant to indicate any device by means of which it is possible to create in a spatial region partially delimited by a dihedral angle, a magnetic field which is substantiallyuniform and parallel to the edge of the dihedral angle; The magnetic prism of FIG. 3 may also be an electromagnet; in which case different deviations of the electron beam might be obtained without a mechanical rotation of the deflector, through modifying the strength of the induction field. However, the use of a permanent magnet eliminates any feeding complication due to the use of an electromagnet which would have to be at the post-acceleration potential.

FIG. 6 shows schematically an embodiment of the electrooptical structure of the detection part of a mass spectrometer of the scintillation type, with an ion-electron converter according to the present invention.

It will readily be appreciated that in this case, the problem, as concerns the magnetic deflector, is very much simplified relatively to the case of an image converter, since there is no need to worry about image distortion.

By way of example, the incident ion beam, filtered as to the charge-to-mass ratio of the ions, will be assumed to issue from a space region at ground potential and to have an energy of about 5 keV when traversing the entrance diaphragm 80, shown in FIG. 6, which is a part of a grounded conductive envelope 82, only a part of which is shown in the FIG.

It will also be assumed that the beam emanates from an optical system such that it shows a crossover 1 before traversing this diaphragm. Under the aforesaid conditions, given merely by way of example to show how the problem of operating the assembly either with positive ions or negative ions may advantageously be solved, the electro-optical structure may be advantageously designed in the following way:

Inside the conductive housing 82, the converter comprises a conductive cylindrical part 50, the two bases of which are provided with apertures andform two further diaphragms 54 and 55 for the incident beam, and thereafter a plane electrode 56, parallel to the bases of the cylinder, building up the secondary electron emitting target. The axis of the apparatus passes through the centers of the successive diaphragms. Inside the cylinder 50, a magnetic deflector, preferably a permanent magnet, which, in this example is supposed to be in a fixed position, generates a magnetic field perpendicular to the plane of the FIG. in the circular hatched region57 thereof, which means that the gap of this magnet has a circular cross section. A plane scintillator 58 obturates an aperture in the wall of the cavity 50. i i

A voltage source 62 of 40 IN is used, the positive and negative terminals thereof being respectively connected to two resistors 71 and 72, having a common terminal 73. The value of the resistor 72 is three times that of the resistor 71.

The positive and negative terminals of the voltage source 62 are also respectively connected to the fixed contacts p and n of a switch 63 whose moving contact is grounded.

When positive ions are used switch.63 is in position p, the terminal 73 and the walls of the cavity 50 are at -l0 kV and the electron emissive target 56 at -40 k\/.

The incident ions are accelerated between the diaphragms and 54 and again between the diaphragm 55 and the target 56, the two former diaphragms forming in addition a converging lens which, on its own, would cause the ion beam to have a further cross over in 1,, beyond the target 56.

The corresponding ion beam P is shown in solid line in the FIG., its virtual prolongation (disregarding the converging action of the lens 5556), beyond the target 56, being shown in dot-dash line.

The secondary electrons emitted by the target 56 are concentrated into a secondary beam, and accelerated, by the 30 kV potential difference between the em'issive target 56 and the diaphragm 54. When entering the region 57 they are deflected at a angle toward the scintillator 58. The adjustment ofthe magnetic field to operate such a deflection ofelectrons with an energy of about 30 keV does not of course appreciably modify the path of the incident ion beam with an energy of +l0= keV.

The electron beam has been shown in El.

The scintillator 58 is conventionally coupled optically by a concentration optical. system 59 to a photomultiplier 61 located behind a porthole 60 provided in the wall 82 of the apparatus.

lf rieg'ative ions are used, the switch 63 is brought to the n position, and the cylinder 50 is brought to +30 kV, while target 56 is grounded. The ionsof the incident beam are acclerated by the potential difference of +30 kV between the diaphragms 82 and 54. The converging action of the lens formed by those two diaphragms being much stronger than in the case of positive ions, the beam shows a real crossover l inside the cylindrical part 50 and diverges again so that at the level of the target 56, it has substantially the same cross section as beam P. The negative ion beam Ng has been shown in the FlG. in dash line.

The decelerating potential of +30 kV for the negative ion beam, between electrodes 55 and 56 still leaves to this beam its initial energy of 5 keV which is sufficient to ensure a secondary electron emission.

As for those secondary electrons, they are submitted to exactly the same conditions as in the case of a positive ion beam, the potential difference between electrode 56 and the entrance diaphragm 80 being again +30 kV.

A prismatic magnet might of course be used here, and also an electromagnet.

The invention is not limited to the embodiments described and shown by way ofexample.

lclaim:

1. An ion-electron converter comprising an envelope having a face with an aperture forming an entrance diaphragm for an incident ion beam and, in said envelope: a target, emitting electrons in response to the impact of ions, said target being located opposite said entrance diaphragm, the straight line normal to said target and passing through the center of said aperture being referred to as the axis of the converter; means, in the vicinity of said target, for concentrating said electrons into a secondary beam whose axis substantially coincides with said converter axis; first electron-responsive means which, as viewed from the point of intersection of said converter axis with said target, are located in a first solid angle which does not include said converter axis; second electron-responsive means which, as viewed from said point of intersection, are located in a second solid angle including neither said converter axis nor any straight line contained in said first solid angle; a magnetic deflector located between said concentrating means and said entrance diaphragm for deflecting said secondary beam; and controlling means coupled to said deflector and extending through said envelope for selectively. causing said deflector to deflect said secondary beam toward said first electron-responsive means .or toward said-second electronresponsive means.

2. An ion-electron converter asc laimed in claim 1, wherein said magnetic deflector is a permanentmagnet designed to generate a magnetic field normal tosaid cpnverter axis, and wherein said controlling means are eclianical means for rotating said permanent magnet'about said converter axis.

3. An ion-electron converter as claimed in claim 1, for converting an ion image into an electron image, wherein said first electron responsive means area luminescent screen and having a face with an aperture forming an entrance diaphragm for an incident lOl'l beam and, in sa d envelope: a target,

emitting electrons in response to theimpact of ions, said target being located opposite said entrance diaphragm, the straight line normal to said target and passing through the center of said aperture being referred to as the axis of the converter; means, in the vicinity of said target, for concentrating said electrons into a secondary beam whose axis substantially coincides with said converter axis; a luminescent screen which, as viewed from the point of intersection of said converter axis with said target, is located in a first solid angle which does not include said converter axis; a carrier ofphotographic emulsion which, as viewed from said point ofinter'section, is located in a second solid angle including neither said converter axis nor any straight line contained in'said first solid angle; a permanent magnet located between said concentrating means and said entrance diaphragm; said converter further comprising means for viewing said luminescent screen, and controlling means coupled to said magnet and extending through said envelope for rotating said magnet about said converter axis, thereby selectively causing said deflector to deflect said secondary beam toward said luminescent screen or toward said carrier of photographic emulsion. 

